Yohimbine dimers exhibiting binding selectivities for alpha2 adrenergic receptors

ABSTRACT

The present invention relates to yohimbine dimer compounds, pharmaceutical compositions containing such dimer compounds, methods of making such dimer compounds, and uses thereof. The yohimbine dimer compounds include compounds of formula (I):  
                 
 
     where R is any linker molecule which affords a yohimbine dimer that has activity as an α 2 -AR antagonist and has selectivity for an α 2 -AR subtype over another α 2 -AR subtype.

[0001] This application is a divisional of U.S. patent application Ser.No. 10/106,521, filed Mar. 25, 2002, which claims the benefit of U.S.Provisional Application Serial No. 60/278,181, filed Mar. 23, 2001, eachof which is hereby incorporated by reference in its entirety.

[0002] This work was supported by the National Institutes of Health,Grant No. NIH R01 GM 29358. The U.S. Government may have certain rightsin this invention.

FIELD OF THE INVENTION

[0003] The present invention relates to yohimbine dimer compounds,pharmaceutical compositions containing such compounds, methods of makingsuch compounds, as well as various uses thereof.

BACKGROUND OF THE INVENTION

[0004] The initial classification of adrenergic receptors (AR's) into α-and β-families was first described by Ahlquist in 1948 (Ahlquist RP, “AStudy of the Adrenergic Receptors,” Am. J. Physiol. 153:586-600 (1948)).Functionally, the α-ARs were shown to be associated with most of theexcitatory functions (vasoconstriction, stimulation of the uterus andpupil dilation) and inhibition of the intestine. On the other hand,β-ARs were implicated in vasodilation, bronchodilation and myocardialstimulation (Lands et al., “Differentiation of Receptor SystemsActivated by Sympathomimetic amines,” Nature 214:597-598 (1967)). Sincethis early work, α-ARs have been subdivided into α₁- and α₂-AR. Cloningand expression of α-AR have confirmed the presence of multiple subtypesof both α₁-(α_(1a), α_(1b), α_(1d)) and α₂-(α_(2a), α_(2b), (α_(2c)) AR(Michel et al., “Classification of a₁-Adrenoceptor Subtypes,”Naunyn-Schmiedeberg's Arch. Pharmacol, 352:1-10 (1995); Macdonald etal., “Gene Targeting—Homing in on α₂-Adrenoceptor-Subtype Function,”TIPS, 18:211-219 (1997)).

[0005] In humans, the three α₂-AR subtypes are encoded by distinct geneslocalized in different chromosomes. The α_(2a)-AR gene is located inchromosome 10, while the α_(2b)-AR gene is found on chromosome 2 and theα_(2c)-AR gene on chromosome 4 (Bylund et al., “International Union ofPharmacology Nomenclature of Adrenoceptors,” Pharmacol. Rev., 46:121-142(1994)).

[0006] Current therapeutic uses of α₂-ARs drugs involve the ability ofthose drugs to mediate many of the physiological actions of theendogenous catecholamines and there are many drugs that act on thesereceptors to control hypertension, analgesia, anesthesia, and ocular andnasal congestion.

[0007] α₂-ARs are found in the rostral ventrolateral medulla, and areknown to respond to the neurotransmitter norepinephrine and theantihypertensive drug clonidine to decrease sympathetic outflow andreduce arterial blood pressure (Bousquet et al., “Role of the VentralSurface of the Brain Stem in the Hypothesive Action of Clonidine,” Eur.J. Pharmacol., 34:151-156 (1975); Bousquet et al., “ImidazolineReceptors: From Basic Concepts to Recent Developments,” 26:S1-S6(1995)). Clonidine and other imidazolines also bind to imidazolinereceptors (formerly called imidazoline-guanidinium receptive sites orIGRS) (Bousquet et al., “Imidazoline Receptors: From Basic Concepts toRecent Developments,” 26:S1-S6 (1995)). Some researchers have speculatedthat the central and peripheral effects of imidazolines as hypotensiveagents may be related to imidazoline receptors (Bousquet et al.,“Imidazoline Receptors: From Basic Concepts to Recent Developments,”26:S1-S6 (1995); Reis et al., “The Imidazoline Receptor: Pharmacology,Functions, Ligands, and Relevance to Biology and Medicine,” Ann. N.Y.Acad. Sci., 763:1-703 (1995); Diamant et al., “Imidazoline Binding Sitesin Human Placenta: Evidence for Heterogeneity and a Search forPhysiological Function,” Br. J. Pharmacol., 106:101-108 (1992);Ragunathan et al., “Imidazoline Receptors and Their Endogenous Ligands,”Annu. Rev. Pharmacol. Toxicol. 36:511-544 (1996); Miralles et al.,“Discrimination and Pharmacological Characterization of I₂-ImidazolineSites with [³H]idazoxan and Alpha-2-Adrenoceptors [³H]RX821002 (2Methoxy Idazoxan) in Human and Rat Brains,” J. Pharmacol. Exp. Ther.,264:1187-1197 (1993)). The pharmacological profiles of drugs acting onimidazoline receptors have resulted in their classification into twomain types: I₁- and I₂-imidazoline binding sites (Diamant et al.,“Imidazoline Binding Sites in Human Placenta: Evidence for Heterogeneityand a Search for Physiological Function,” Br. J. Pharmacol., 106:101-108(1992); Miralles et al., “Discrimination and PharmacologicalCharacterization of I₂-Imidazoline Sites with [³H]idazoxan andAlpha-2-Adrenoceptors [³H]RX821002 (2 Methoxy Idazoxan) in Human and RatBrains,” J. Pharmacol. Exp. Ther., 264:1187-1197 (1993); Olmos et al.,“Pharmacological and Molecular Discrimination of Brain I₂-ImidazolineReceptor Subtypes,” Naunyn-Schmiedberg's Arch. Pharmacol. 354:709-716(1996)).

[0008] Yohimbine is a known potent and selective α₂-AR antagonist, andhas been used extensively as a pharmacological probe for studying theα₂-AR (Starke K., Rev. Physiol. Biochem. Pharmacol, 88, 199 (1981)).Yohimbine, an indole alkaloid isolated from Pausinystlia yohimbe barkand Rauwolfia roots, is an α₂-antagonist selective for α₂ over α₁adrenoreceptors, but is also a serotonic antagonist. It has actions bothin the central nervous system and in the periphery inducing hypertensionand increases heart rate. Yohimbine has been used to treat maleimpotence and posturalo hypotension. It has also been used in researchto induce anxiety (Foye et al. Principles of Medicinal Chemistry, FourthEdition, Williams & Wilkins (1995), page 359). However, yohimbine doesnot show selectivity among three α₂-AR subtypes (Hieble et al., J. Med.Chem., 38, 3415 (1995); Hieble et al., Prog. Drug Res., 47, 81 (1996)).

[0009] Although there have been a number of α₂-AR antagonists identified(Ruffolo et al., J. Med. Chem. 38, 3681 (1995); Clark et al., Prog. Med.Chem. 23, 1 (1986)), only a small set of compounds have been reportedthat have a varied degree of selectivity among the three subtypes ofα₂-AR. However, these latter compounds suffer from either low subtypeselectivity or binding to receptor sites outside the α₂-AR subfamily(Ruffolo et al., J. Med. Chem. 38, 3681 (1995; Yound et al, Eur. J.Pharmacol., 168, 381 (1989); Devedjian et al, Eur. J. Pharmacol., 252,43 (1994); Meana et al., Eur. J. Pharmacol., 312, 385 (1996); Beeley etal., Bioorg. Med. Chem., 3, 1693 (1995); Michel et al., Br. J.Pharmacol., 99, 560 (1990); Uhlen et al., Pharmacol. Exp. Ther., 271,1558 (1994); Blaxall et al., Pharmacol. Exp. Ther., 259, 353 (1991);Bylund et al., Mol. Pharmacol., 42, 1 (1992); Okumura et al, Gen.Pharmacol. 19, 463 (1988)). Thus, a need exists to identify compoundswhich bind α₂-AR subtypes with sufficient affinity and selectivity.

[0010] The present invention is directed to overcoming these and otherproblems encountered in the art.

SUMMARY OF THE INVENTION

[0011] The present invention relates to a compound according to formula(I):

[0012] wherein R is a linker molecule, preferably having a length ofabout 2.5 Å to about 45 Å.

[0013] The present invention also relates to a compound according toformula (I), wherein R (i.e., as a linker molecule) is a straight orbranched chain alkyl, alkenyl, alkynyl comprising at least 2 carbonatoms in a main chain thereof; wherein R (i.e., as a linker molecule) isa straight or branched chain alkyl, alkenyl, alkynyl comprising at least2 carbon atoms in a main chain thereof and an X group within the mainchain and/or a Y group as a substituent linked to a carbon atom in themain chain with X being —O—, carbonyl, —NR¹— with R¹ being H or analkyl, —C(O)NHR¹— with R¹ being an alkyl, —S—, sulfoxide, sulfonyl, or acyclic or multicyclic ring with or without heteroatoms as ring membersand including, optionally, one or more substitutions on the ringstructure(s), and with Y being —OH, —NO₂, —CN, —C(O)H, —SH, or aprimary, secondary, or tertiary amine, a carboxylic acid, an ester, aketo group, —SO₂NH₂, or —SO₂NHR² with R² being an alkyl; or wherein R(i.e., as a linker molecule) is a cyclic or multicyclic ring with orwithout hetero atoms as ring members and including, optionally, one ormore substitutions on the ring structure(s).

[0014] The present invention also relates to a pharmaceuticalcomposition, which includes a yohimbine dimer of the present inventionand a pharmaceutically acceptable carrier.

[0015] The present invention further relates to a method of making ayohimbine dimer of the present invention, which includes reactingyohimbic acid with a di-amine according to the formula H₂N—R—NH₂ underconditions effective to yield a yohimbine dimer of the presentinvention.

[0016] Yet another aspect of the present invention relates to a methodof treating or preventing an α₂ adrenergic receptor mediated conditionor disorder which includes: administering to a patient an effectiveamount of a yohimbine dimer of the present invention under conditionseffective to treat or prevent the α₂ adrenergic receptor mediatedcondition or disorder.

[0017] A further aspect of the present invention relates to a method ofmodulating the activity of α_(2a) adrenergic receptor which includes:contacting an α_(2a) adrenergic receptor with a yohimbine dimer of thepresent invention under conditions effective to modulate the activity ofthe α_(2a) adrenergic receptor.

[0018] A still further aspect of the present invention relates to amethod of modulating the activity of an α_(2c) adrenergic receptor whichincludes: contacting an α_(2c) adrenergic receptor with a yohimbinedimer of the present invention under conditions effective to modulatethe activity of the α_(2c) adrenergic receptor.

[0019] The present invention provides additional advantages because itidentifies a bivalent ligand approach to identify (α₂-ARsubtype-selective antagonists. As demonstrated herein, the bivalentyohimbine ligands exhibit a higher degree of potency, binding affinity,and selectivity than their monovalent counterparts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 shows a graph of P_(ki) values versus dimer length foryohimbine analogs.

[0021]FIG. 2 shows a graph illustrating microvessel responses to vehicleand yohimbine administration. The first 180 sec represents the responseto vehicle injection and the arrow indicates the time point associatedwith yohimbine injection. *=P<0.05, n=6.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention relates to a compound according to formula(I):

[0023] where R is any linker molecule which affords a yohimbine dimerthat has activity as an α₂-AR antagonist and, specifically, hasselectivity for one α₂-AR subtype over another α₂-AR subtype.Preferably, the yohimbine dimer compounds of the present inventioneither have selectivity for (i) the α_(2a)-AR over the α_(2b)-AR, or(ii) the α_(2c)-AR over the α_(2b)-AR. Depending on the length of thelinker molecule, selectivity between the α_(2c)-AR and α_(2a)-AR canalso be achieved.

[0024] According to one aspect of this invention, R is a linkermolecule, wherein R is (i) a straight or branched chain alkyl, alkenyl,alkynyl including at least 2 carbon atoms in a main chain thereof, (ii)a straight or branched chain alkyl, alkenyl, alkynyl comprising at least2 carbon atoms in a main chain thereof and an X group within the mainchain and/or a Y group as a substituent linked to a carbon atom in themain chain, with X being—O—, carbonyl, —NR¹— with R¹ being H or analkyl, —C(O)NHR¹— with R¹ being an alkyl, —S—, sulfoxide, sulfonyl, or acyclic or multicyclic ring with or without hetero atoms as ring membersand including, optionally, one or more substitutions on the ringstructure(s) and with Y being —OH, —NO₂, —CN, —C(O)H, —SH, or a primary,secondary, or tertiary amine, a carboxylic acid, an ester, a keto group,—SO₂NH₂, or —SO₂NHR² with R² being an alkyl; or (iii) a cyclic ormulticyclic ring with or without hetero atoms as ring members andincluding, optionally, one or more substitutions on the ringstructure(s).

[0025] According to a second aspect, R is a linker molecule having alength of about 2.5 Å to about 45 Å, more preferably within a firstrange of about 2.5 Å to about 5 Å or a second range of about 23 Å toabout 29 Å.

[0026] In accordance with the present invention, suitable chemicalstarting materials, such as different reactants, reagents, such ascompounds with different chemical functional substituent groups, thatcombine to form the bivalent yohimbine dimer compounds of formula (I)are defined below. Such reactant compounds include, without limitation,yohimbic acid and its corresponding derivatives.

[0027] Suitable for use in the present invention are a yohimbinecompound, which is conventionally known in the literature as a indolealkaloid and any corresponding yohimbine derivatives (i.e., substitutedwith different chemical group substituents) as prepared or synthesizedby known organic synthetic methods. Preferably the yohimbine compound(or its derivative) is in the form of a yohimbic acid.

[0028] Yohimbic acid, as noted hereinafter, is reacted with a diamineH₂N—R—NH₂, where R is a linker molecule of the type described above.

[0029] In the present invention, R and/or R¹ are defined as straight orbranched chain alkyls, which include at least 2 carbons in a main chainthereof. Straight chain alkyl groups which are represent by the formula—(CH₂)_(n)—, which is a methylene unit functional group fragment, wheren is an integer of greater than or equal to 2 units (i.e. n≧2). Branchedchain alkyls may have the formula as defined above for a straight chainalkyl, except that one or more CH₂ groups may be replaced by —CHW)_(n)—groups wherein W represents an alkyl side chain.

[0030] A preferred embodiment for chain representations is where theterm n is defined as being an integer of at least 2, preferably n is aninteger from at least 2 to 36 units, more preferably from 3 to 36 units.In alternate preferred embodiments of the present invention, compoundsof the present invention have straight or branched alkyl groups, whereinn is an integer greater than or equal to 18, more preferably greaterthan or equal to an integer from 24 to 36.

[0031] In the present invention, R may be defined as a straight orbranched chain alkenyl group, which includes at least 5 carbons in amain chain thereof. In accordance with the present invention, straightchain alkenyls may be represented by the formula—CH₂)_(xa)CH═CH(CH₂)_(xb)— where xa and xb each are independentlydefined as integers that may be greater than or equal to the integerzero (i.e., xa and xb≧0), xa and xb being the same or different.Branched chain alkenyls have the formula as defined above for straightchain alkenyl, except that one or more CH₂ groups, may be replaced byCHW groups or a CH group is replaced by a CW group, where W is an alkylside chain.

[0032] In the present invention, R may also be defined as a straight orbranched chain alkynyl group, which includes at least 5 carbon atoms ina main chain thereof. Suitable for use in the present invention,straight chain alkynyl groups are represented by the formula—(CH₂)_(xc)C≡C(CH₂)_(xd)— where xc and xd each are independently definedas integers that may be greater than or equal to the integer zero (i.e.,xc and xd≧0), with xc and xd being the same or different. In addition,branched chain alkynyls have the formula as defined above for straightchain alkynyl groups, except that one or more CH₂ groups are replaced byCHW groups, where W is an alkyl side chain, i.e., as represented by theformula—(CHW)_(xc)C≡C(CHW)_(xd)—.

[0033] In accordance with the present invention, R may be a straight orbranched chain alkyl, alkenyl, alkynyl with at least 2 carbon atoms in amain chain thereof and an X group within the main chain and/or a Y groupas a substituent linked to a carbon atom in the main chain. X can be—O—, carbonyl, —NR¹— with R¹ being an alkyl, —C(O)NHR¹— with R¹ being analkyl, —S—, sulfoxide, sulfonyl, or a cyclic or muticyclic ring with orwithout hetero atoms as ring members and including, optionally, one ormore substitutions on the ring structure(s). Y can be —OH, —NO₂, —CN,—C(O)H, —SH, a primary, secondary, or tertiary amine, a carboxylic acid,an ester, a keto group, —SO₂NH₂, or —SO₂NHR² with R² being an alkyl.

[0034] In addition, R can be a cyclic or multicyclic ring with orwithout hetero atoms as ring members and including, optionally, one ormore substitutions on the ring structures.

[0035] Cyclic or multicyclic rings (whether defined as R or as X above)can be aromatic or non-aromatic, either with or without N, S, or Ohetero ring members, including without limitation, cycloalkanes,cycloalkenes, phenyls, indenes, pyrroles, imidazoles, oxazoles,pyrrazoles, pyridines, pyrimidines, pyrrolidines, piperidines,thiophenes, furans, napthals, bi-phenyls, indoles, tetrahydrofurans,decalins, and tetrahydrothiophenes.

[0036] The cyclic or multicyclic rings suitable for use in the presentinvention can include mono-, di-, tri or multiple additionalsubstitutions of each different ring type, where standard organicchemistry principles, such as valency permit. In the case of aromatic,heteroaromatic rings and the like, additional chemical functional groupsubstituents may include mono-, di-, or tri-substitutions of eachdifferent ring type, which may be located at the ortho, meta, orparapositions on such cyclic ring systems relative to where the ring ispositioned within a R group substituent. An exemplary aromatic cyclicgroup bearing alkyl-substitutions at para positions is

[0037] In accordance with the present invention, additional functionalgroup substitutions on such ring systems may include, withoutlimitation, alkyl, alkoxy, acylalkyl, halogen, mercapto, nitro, hydroxy,aldehyde, carboxylic acids, primary, secondary, or tertiary amines, andketones.

[0038] According to one embodiment, a bivalent yohimbine dimer compoundof the present invention includes an R group which may be a straightchain alkyl comprising at least 4 carbon atoms in a main chain thereofand an X group within the main chain, wherein X is a —O—. Such an Rgroup may be represented by —(CH₂—CH₂—O—CH₂—CH₂)_(n)— where n is aninteger from 1 to 6. For example, an alkyl group bearing an X group inthe main chain is —(CH₂)₄—O—(CH₂)₄—. Another example is represented by—(CH₂—CH₂—O—CH₂—CH₂—O—CH₂—CH₂)_(n)— wherein n is an integer from 1 to 4.Moreover, an exemplary R group that is an alkyl group bearing a Y groupas a substituent is —(CH₂)₄—CH(OH)—(CH₂)₄—.

[0039] In another embodiment, a bivalent yohimbine dimer compound of thepresent invention includes an R group which is a straight chain alkylcomprising at least 5 carbon atoms in a main chain thereof and an Xgroup within the main chain, wherein X is —C(O)NHR¹— with R¹ being analkyl. Such an R group can be represented by—CH₂—NHC(O))_(n)—CH₂—CH₂—CH₂—CH₂—(C(O)NH—CH₂)_(n)— wherein each n isindependently an integer from 1 to 3.

[0040] In another embodiment of the present invention, a bivalentyohimbine dimer compound includes an R group that is a cis isomer or atrans isomer of —CH₂—NHC(O))_(n)—CH₂—CH═CH—CH₂—(C(O)NH—CH₂)_(n)— whereineach n is independently an integer from 1 to 3. An R group representedby the cis isomer is

[0041] wherein each n is independently an integer from 1 to 3. An Rgroup represented by the trans isomer is

[0042] wherein each n is independently an integer from 1 to 3.

[0043] Another aspect of the present invention relates to a method ofmaking a yohimbine dimer, which includes reacting yohimbic acid and/orits corresponding derivatives with a di-amine and/or its correspondingderivatives according to the formula H₂N—R—NH₂ under conditionseffective to yield a yohimbine dimer according to formula (I) aspreviously described. In particular, an initial reactant for forming theyohimbine dimer of the present invention, a compound which is a diaminehaving the formula H₂N—R—NH₂ is needed, where R is the same defined asabove.

[0044] Suitable conditions effective to yield a yohimbine dimeraccording to Formula I of the present invention, include the use ofconventional starting materials, reagents, solvents and/or reactionconditions, etc. as described below.

[0045] Many such diamines are commercially available or otherwisereadily synthesizable according to conventional art known syntheticorganic chemical techniques and/or procedures. For example, commerciallyavailable diamines include, without limitation, ethylene diamine,1,2-propane diamine, 1,3-pentane diamine, 1,6-hexane diamine,p-phenylene diamine, napthalene 2,6-diamine, 2,4-toluene diamine,piperazine, 4,4′ bis(dimethylamino)benzophenone, spermidine, putricine,cadaverine, or other natural polyamines.

[0046] According to one approach, standard peptide coupling procedurescan be employed. For example, commercially available yohimbic acid canbe reacted with aliphatic diamines with the intervening methylene groupsranging from about 2 to about 36 under standard peptide coupling(DCC,HOBT) conditions.

[0047] The method for the synthesis of compounds of the presentinvention can be carried out in an organic solvent. Conventional organicsolvents can be used, which include, but are not limited to toluene,methyl-tert-butyl ether, pyridine, chloroform, methylene chloride,acetonitrile, N,N-dimethyl formamide (“DMF”), glymes, tetrahydrofuran(“THF”), isooctane, and mixtures of these solvents.

[0048] Alternatively, a variety of amide formation processes can beemployed, using acid halides, carboxylic acid anhydrides, or carboxylicacids with heat and appropriate amines or diamines.

[0049] The reaction of the present invention can be carried out at arange of suitable temperatures, such as from 0° C. to 150° C., morepreferably at 0° C. to 80° C. Optimal reaction temperatures can beselected based upon the particular choices of solvents and reactants.

[0050] The compounds prepared by the methods of the present inventioncan be pharmaceutically acceptable salts in the form of inorganic ororganic acid or base addition salts of the above compounds. Suitableinorganic acids are, for example, hydrochloric, hydrobromic, sulfuric,and phosphoric acids. Suitable organic acids include carboxylic acids,such as, acetic, propionic, glycolic, lactic, pyruvic, malonic,succinic, fumaric, malic, tartaric, citric, cyclamic, ascorbic, maleic,hydroxymaleic, dihydroxymaleic, benzoic, phenylacetic, 4-aminobenzoic,anthranilic, cinnamic, salicylic, 4-aminosalicylic, 2-phenoxybenzoic,2-acetoxybenzoic, and mandelic acid. Sulfonic acids, such as,methanesulfonic, ethanesulfonic, and α-hydroxyethane-sulfonic acid arealso suitable acids. Non-toxic salts of the compounds of theabove-identified formulae formed with inorganic and organic basesinclude, for example, those alkali metals, such as, sodium, potassium,and lithium, alkaline earth metals, for example, calcium and magnesium,light metals of group IIIA, for example, aluminum, organic amines, suchas, primary, secondary, or tertiary amines, for example,cyclohexylamine, ethylamine, pyridine, methylaminoethanol, andpiperazine.

[0051] Another aspect of the present invention relates to apharmaceutical composition, which includes a yohimbine dimer asdescribed above and a pharmaceutically acceptable adjuvant, carrier,and/or excipient.

[0052] The compounds or compositions prepared according to the presentinvention can be used to treat warm blooded animals, such as mammals.Examples of such beings include, without limitation, humans, cats, dogs,horses, sheep, cows, pigs, lambs, rats, mice, and guinea pigs.

[0053] Conventional administration methods may be suitable for use inthe present invention as described below.

[0054] Compounds or compositions within the scope of this inventioninclude all compounds or compositions, wherein the compound of thepresent invention is contained in an amount effective to achieve itsintended purpose. While individual needs vary, determination of optimalranges of effective amounts of each component is within the skill of theart. The quantity of the compound or composition administered will varydepending on the patient and the mode of administration and can be anyeffective amount. Typical dosages include about 0.01 to about 100mg/kg·body wt. The preferred dosages include about 0.01 to about 0.1mg/kg·body wt up to three times a day. Treatment regimen for theadministration of the compounds of the present invention can also bedetermined readily by those with ordinary skill in art. The quantity ofthe compound administered may vary over a wide range to provide in aunit dosage an effective amount of from about 0.01 to 20 mg/kg of bodyweight of the patient per day to achieve the desired effect. Forexample, the desired antihistamine, antiallergy, and bronchodilatoreffects can be obtained by consumption of a unit dosage form such as atablet containing 1 to 50 mg of the compound of the present inventiontaken 1 to 4 times daily.

[0055] The compounds prepared by the methods of the present inventioncan be utilized as the biologically active components in pharmaceuticalcompositions.

[0056] Depending upon the treatment being effected, the compounds orcompositions of the present invention can be administered orally,topically, transdermally, parenterally, subcutaneously, intravenously,intramuscularly, intraperitoneally, by intranasal instillation, byintracavitary or intravesical instillation, intraocularly,intraarterially, intralesionally, or by application to mucous membranes,such as, that of the nose, throat, and bronchial tubes.

[0057] The yohimbine dimer compounds and/or pharmaceutical compositioncan also include, but are not limited to, suitable adjuvants, carriers,excipients, or stabilizers, and can be in solid or liquid form such as,tablets, capsules, powders, solutions, suspensions, or emulsions.Typically, the composition will contain from about 0.01 to 99 percent,preferably from about 20 to 75 percent of active compound(s), togetherwith the adjuvants, carriers and/or excipients. For example, applicationto mucous membranes can be achieved with an aerosol spray containingsmall particles of a compound of this invention in a spray or dry powderform.

[0058] The solid unit dosage forms can be of the conventional type. Thesolid form can be a capsule and the like, such as an ordinary gelatintype containing the compounds of the present invention and a carrier,for example, lubricants and inert fillers such as, lactose, sucrose, orcornstarch. In another embodiment, these compounds are tableted withconventional tablet bases such as lactose, sucrose, or cornstarch incombination with binders like acacia, cornstarch, or gelatin,disintegrating agents, such as cornstarch, potato starch, or alginicacid, and a lubricant, like stearic acid or magnesium stearate.

[0059] The tablets, capsules, and the like can also contain a bindersuch as gum tragacanth, acacia, corn starch, or gelatin; excipients suchas dicalcium phosphate; a disintegrating agent such as corn starch,potato starch, alginic acid; a lubricant such as magnesium stearate; anda sweetening agent such as sucrose, lactose, or saccharin. When thedosage unit form is a capsule, it can contain, in addition to materialsof the above type, a liquid carrier such as a fatty oil.

[0060] Various other materials may be present as coatings or to modifythe physical form of the dosage unit. For instance, tablets can becoated with shellac, sugar, or both. A syrup can contain, in addition toactive ingredient, sucrose as a sweetening agent, methyl andpropylparabens as preservatives, a dye, and flavoring such as cherry ororange flavor.

[0061] For oral therapeutic administration, these active compounds canbe incorporated with excipients and used in the form of tablets,capsules, elixirs, suspensions, syrups, and the like. Such compositionsand preparations should contain at least 0.1% of active compound. Thepercentage of the compound in these compositions can, of course, bevaried and can conveniently be between about 2% to about 60% of theweight of the unit. The amount of active compound in suchtherapeutically useful compositions is such that a suitable dosage willbe obtained. Preferred compositions according to the present inventionare prepared so that an oral dosage unit contains between about 1 mg and800 mg of active compound.

[0062] The active compounds of the present invention may be orallyadministered, for example, with an inert diluent, or with an assimilableedible carrier, or they can be enclosed in hard or soft shell capsules,or they can be compressed into tablets, or they can be incorporateddirectly with the food of the diet.

[0063] The pharmaceutical forms suitable for injectable use includesterile aqueous solutions or dispersions and sterile powders for theextemporaneous preparation of sterile injectable solutions ordispersions. In all cases, the form should be sterile and should befluid to the extent that easy syringability exists. It should be stableunder the conditions of manufacture and storage and should be preservedagainst the contaminating action of microorganisms, such as bacteria andfungi. The carrier can be a solvent or dispersion medium containing, forexample, water, ethanol, polyol (e.g., glycerol, propylene glycol, andliquid polyethylene glycol), suitable mixtures thereof, and vegetableoils.

[0064] The compounds or pharmaceutical compositions of the presentinvention may also be administered in injectable dosages by solution orsuspension of these materials in a physiologically acceptable diluentwith a pharmaceutical adjuvant, carrier or excipients. Such adjuvants,carriers and/or excipients, include, but are not limited to sterileliquids, such as water and oils, with or without the addition of asurfactant and other pharmaceutically and physiologically acceptablecarrier, including adjuvants, excipients or stabilizers. Illustrativeoils are those of petroleum, animal, vegetable, or synthetic origin, forexample, peanut oil, soybean oil, or mineral oil. In general, water,saline, aqueous dextrose and related sugar solution, and glycols, suchas propylene glycol or polyethylene glycol, are preferred liquidcarriers, particularly for injectable solutions.

[0065] These active compounds may also be administered parenterally.Solutions or suspensions of these active compounds can be prepared inwater suitably mixed with a surfactant such as hydroxypropylcellulose.Dispersions can also be prepared in glycerol, liquid polyethyleneglycols, and mixtures thereof in oils. Illustrative oils are those ofpetroleum, animal, vegetable, or synthetic origin, for example, peanutoil, soybean oil, or mineral oil. In general, water, saline, aqueousdextrose and related sugar solution, and glycols such as, propyleneglycol or polyethylene glycol, are preferred liquid carriers,particularly for injectable solutions. Under ordinary conditions ofstorage and use, these preparations contain a preservative to preventthe growth of microorganisms.

[0066] For use as aerosols, the compounds of the present invention insolution or suspension may be packaged in a pressurized aerosolcontainer together with suitable propellants, for example, hydrocarbonpropellants like propane, butane, or isobutane with conventionaladjuvants. The materials of the present invention also may beadministered in a non-pressurized form such as in a nebulizer oratomizer.

[0067] The present invention also relates to modulating the activity ofan α₂-AR, preferably selectively modulating the activity of α_(2a)-AR orα_(2c)-AR versus α_(2b)-AR. This aspect of the present invention can beachieved by contacting an α_(2a) or α_(2c) adrenergic receptor with ayohimbine dimer of the present invention under conditions effective tomodulate the activity of the α_(2a) or α_(2c) adrenergic receptor,respectively.

[0068] In so modulating the activity of an α_(2a) or α_(2c) adrenergicreceptor, it is possible to treat or prevent an α₂ adrenergic receptormediated condition or disorder of the type listed below. This can beachieved by administering to a patient an effective amount of ayohimbine dimer of the present invention under conditions effective totreat or prevent the α₂ adrenergic receptor mediated condition ordisorder. More specifically, to treat or prevent disorders or conditionswhich involve the hyperactivity of α_(2a)-AR or activity of normallysilent α_(2c)-AR, these ARs can be selectively targeted relative to theα_(2b)-AR. As a result, numerous side-effect associated with α_(2b)-ARantagonism can be avoided. Concepts pertaining to such methods aredescribed below.

[0069] Suitable conditions for the modulation of the α₂ adrenergicreceptor relate to the fact that α₂ receptors are located bothpre-synaptically at nerve terminals and post-synaptically as in vascularsmooth muscles, platelets, pancreatic β-cells, and fat cells. Activationof the presynaptic receptors inhibit the release of norepinephrine by anegative feedback mechanism. Blockade of these receptors would thereforeincrease the release of norepinephrine.

[0070] Yohimbine is known to be an antagonist of α₂₋AR. This inventionis directed to yohimbine dimer compounds which are selective antagonistsfor human α₂ receptors, showing excellent selectivity between either theα_(2a) or α_(2c) receptors and the α_(2b) receptors. Depending on theparticular R group, selectivity between α_(2a) and α_(2c) receptors canalso be achieved. Thus, the yohimbine dimers of the present inventionare particularly well suited for selectively inhibiting α_(2a)-ARactivity, selectively inhibiting α_(2c)-AR activity, or both. As aresult, the present invention also relates to the use of these compoundsfor treating or preventing disorders which implicate hyperactivity ofα_(2a) adrenergic receptors or the activity of normally silenced α_(2c)adrenergic receptors.

[0071] The α_(2a) adrenergic receptors have been implicated inhyper-/hypotension, pain, glaucoma, alcohol and drug withdrawal,rheumatoid arthritis, ischemia, migraine, cognitive deficiency,spasticity, diarrhea, and nasal congestion. Lakhlani et al.,“Substitution of a Mutant α_(2a)-adrenergic Receptor via ‘Hit and Run’Gene targeting Reveals the Role of this Subtype in Sedative, Analgesic,and Anesthetic-sparing Responses In Vivo,” Proc Natl Acad Sci USA94(18):9950-5 (1997), which is hereby incorporated by reference in itsentirety, provides definitive evidence that the α_(2a) adrenergicreceptor subtype is the primary mediator of clinically important centralactions of α₂ adrenergic receptor agonists. MacMillan et al., “CentralHypotensive Effects of α_(2a)-adrenergic Receptor Subtype,” Science273(5276): 801-3 (1996), which is hereby incorporated by reference inits entirety, demonstrates that the α_(2a) adrenergic receptor subtypeplays a principal role in the hypotensive response to α₂ adrenergicreceptor agonists. Likewise, Makaritsis et al., “SympathoinhibitoryFunction of the α _(2a)-adrenergic Receptor Subtype,” Hypertension34(3):403-407 (1999), which is hereby incorporated by reference in itsentirety, demonstrates that the α_(2a) adrenergic receptor subtypeexerts a sympathoinhibitory effect, and its loss leads to ahypertensive, hyperadrenergic state. Ongoing work by Limbird hasdemonstrated that the α_(2a-)subtype plays a role in suppression ofepileptogenesis, pain perception, sedation and anesthesia, control ofblood pressure by the central nervous system, and depression/sensitivityto antidepressant agents.

[0072] In particular, two related disorders which can be treated orprevented by the α_(2a) adrenergic receptor antagonists of the presentinvention are hypotension (low blood pressure) and hypertension (highblood pressure). By targeting peripheral α_(2a) adrenergic receptors,the yohimbine dimers of the present invention can be used to treathypertension. In contrast, by targeting α_(2a) adrenergic receptors ofthe central nervous system, the yohimbine dimers of the presentinvention can be used to treat hypotension.

[0073] Moreover, consistant with the use of yohimbine to treat erectiledysfunction in males, the compounds of the present invention can also beused to effect such treatment.

[0074] The α_(2c) adrenergic receptors have been implicated in Raynaud'sdisease. The remarkable role of α_(2c)-ARs in vascular dysfunction hasvery recently been discovered. Analysis of cutaneous arteries at 37° C.confirms that α_(2c)-ARs do not contribute to vasoconstriction. However,Flavahan and coworkers (Chotani et al., “Silent Alpha2c-AdrenergicReceptors Enable Cold-Induced Vasoconstriction in Cutaneous Arteries,”Heart and Circulatory Physiology 278:H1075-H1083 (2000); Flavahan etal., “Increased Alpha2-Adrenergic Constriction of Isolated Arterioles inDiffuse Scleroderma,” Arthritis and Rheumatism, 43:1886-1890 (2000),each of these references is hereby incorporated by reference in itsentirety) have shown that during cold-induced vasoconstriction (28° C.),the α_(2c)-ARs are “no longer silent” and are proposed to be responsiblefor the vasospastic episodes in Raynaud's disease.

[0075] Recent studies in the mouse tail artery confirm the previousobservations that the α_(2c)-ARs are activated at lower temperatures(Chotani et al., “Silent Alpha2c-Adrenergic Receptors EnableCold-Induced Vasoconstriction in Cutaneous Arteries,” Heart andCirculatory Physiology, 278:H 1075-H1083 (2000), which is hereinincorporated by reference in its entirety). At 37° C., thevasoconstriction is mediated by ARs (α_(2a)- and (α_(2b)-) while(α_(2c)-ARs are not, or minimally, involved. However, in a remarkableway, the augmented vasoconstrictor response at 28° C. to catecholaminesis mediated primarily by the α_(2c)-AR. This work implies thatα_(2c)-ARs are “silent” at 37° C., but are activated during cold-inducedexposures, e.g. at 28° C.

[0076] The Flavahan group reported that cold induced vasoconstriction isrelated to an increased distribution of α_(2c)-ARs from the Golgiapparatus to the cellular membranes (Jeyaraj et al., “Cooling EvokesRedistribution of Alpha2C-Adrenoceptors from Golgi to Plasma Membrane inTransfected Human Embryonic Kidney 293 Cells,” Mol. Pharmacol.60:1195-200 (2001), which is herein incorporated by reference in itsentirety). The mechanism that silences or suppresses the actions of theα_(2c)-ARs at 37° C. is unknown. The specific temperature effect on theα_(2c)-ARs could reflect altered membrane targeting or processing of the(α_(2c)-ARs or variations in the signaling or amplification process.

[0077] The “Raynaud's Disease Phenomenon” results from the vasospasms inthe digital arterioles in response to cold, causing a sharp demarcatedcutaneous pallor and cyanosis of the digits (Ekenvall et al.,“Alpha-Adrenoceptors and Cold Induced Vasoconstriction in Human FingerSkin,” Am. J. Physiol., 255:H1000-H1003 (1988), which is hereinincorporated by reference in its entirety). Studies with human patientsshow that vascular dysfunction is an important early defect in systemicsclerosis (scleroderma) which occurs prior to tissue fibrosis Flavahanet al., “Increased Alpha2-Adrenergic Constriction of Isolated Arteriolesin Diffuse Scleroderma,” Arthritis and Rheumatism, 43:1886-1890 (2000),which is herein incorporated by reference in its entirety).

[0078] In 95% of the patients with Raynaud's disease, the earliestmanifestation of systemic sclerosis is a vasospastic response of thedigital arteries to cold exposure. A blood vessel wall defect associatedwith Raynaud's disease was postulated by Lewis in 1929 (Lewis et al.,“Experiments Relating to the Peripheral Mechanism Involved in SpasmodicArrest of the Circulation in Fingers: A Variety of Raynaud's Disease,”Heart, 15:7-101 (1929), which is herein incorporated by reference in itsentirety). Thus, the α_(2c)-ARs are not functionally responsive at 37°C., i.e. silent in normal regulation of the vascular function, but areactivated by catecholamines at 28° C.

[0079] Stable cell lines expressing the human α₂ adrenergic receptorsdescribed above as well as stable cell lines expressing the human α₁adrenergic receptors have been deposited with the American Type CultureCollection (ATCC), 12301 Parklawn Drive, Rockville, Md., 20852, U.S.A.,under the provisions of the Budapest Treaty for the InternationalRecognition of the Deposit of Microorganisms for the Purposes of PatentProcedure. The cell line expressing the human α_(2a) receptor isdesignated L-α_(2a) and was deposited on Nov. 6, 1992, under ATCCAccession Number CRL-11180. The cell line expressing the human α_(2b′)receptor is designated L-NGC-α_(2b) and was deposited on Oct. 25, 1989under ATCC Accession Number CRL-10275. The cell line expressing thehuman α_(2c) receptor is designated L-α_(2c) and was deposited on Nov.6, 1992, under ATCC Accession Number CRL-11181.

EXAMPLES

[0080] The Examples set forth below are for illustrative purposes onlyand are not intended to limit, in any way, the scope of the presentinvention.

[0081] Example 1

Synthesis of Yohimbine Dimers

[0082] The designed yohimbine dimers (3a-j) were prepared by couplingcommercially available yohimbine acid with aliphatic α,ω-diamines understandard peptide coupling condition as shown in the chemical reactionsequence below.

[0083] (The structures of bivalent yohimbines were confirmed byspectroscopic analysis (MS, IR. ¹H NMR. ¹³C NMR) and elemental analysis.Due to the conformationally rigid nature of the yohimbine polycyclicring system, (Morrison, G. A., Fortscher. Chem. Organ. Naturstoffe,25:269 (1967), which is herein incorporated by reference in itsentirety), the stereo centers that are susceptible to epimerizationduring the coupling reaction and the subsequent reaction work up arethose at C16 and C17. However, the integrity of their stereochemistrywas conserved as confirmed by decoupling NMR studies. Example 2

Binding Selectivities for Yohimbine Dimers on Human α_(2a-) versusα_(2b-) Adrenergic Receptors

[0084] The bivalent ligand approach has been successfully utilized indeveloping highly potent and selective ligands in a diverse set ofreceptor systems (Portoghese et al., “Stereostructure-activityRelationship of Opioid Agonist and Antagonist Bivaient Ligands: Evidencefor Bridging Between Vicinal Opioid Receptors,” J. Med. Chem.,28:1140-1141 (1985); Erez et al., “Narcotic Antagonistic Potency ofBivalent Ligands Which Contain Beta-naltrexamine: Evidence for Bridgingthe Proximal Recognition Sites,” J. Med. Chem., 25:847-849 (1982);Portoghese, “The Role of Concepts in Structure-activity RelationshipStudies of Opioid Ligands,” J. Med. Chem. 35:1927-1937 (1992);Portoghese, “Bivalent Ligands and the Message-address Concept in theDesign of Selective Opioid Receptor Antagonists,” TIPS 10:230-235(1989); Shimohigashi et al., “Dimeric Tetrapeptide Enkephalins DisplayExtraordinary Selectivity for the Delta Opiate Receptor,” Nature,297:333-335 (1982); LeBoulluec et al., “Bivalent Indoles ExhibitingSerotonergic Binding Affinity,” Bioorg. Med. Chem. Lett., 5:123-126(1995); Cwirla et al., “Peptide Agonist of the Thrombopoietin Receptoras Potent as the Natural Cytokine,” Science, 276:1696-1699 (1997), eachof these references is hereby incorporated by reference in itsentirety), such as the opioid and serotonergic receptors, two members ofthe seven transmembrane G protein-coupled receptor super-family, as wellas the growth factor receptor system.

[0085] In these studies of the present invention, it was demonstratedthat bivalent ligands exhibit a higher degree of potency and selectivitythan their monovalent counterparts. This superior activity of bivalentligands was shown to result from the bridging between either vicinalreceptors or the pharmacophore binding site and another accessory sitein the same receptor molecule (Portoghese et al.,“Stereostructure-activity Relationship of Opioid Agonist and AntagonistBivalent Ligands: Evidence for Bridging Between Vicinal OpioidReceptors,” J. Med. Chem. 28:1140-1141 (1985); Erez et al., “NarcoticAntagonistic Potency of Bivalent Ligands Which ContainBeta-naltrexamine: Evidence for Bridging the Proximal RecognitionSites,” J. Med. Chem., 25:847-849 (1982); Portoghese, “The Role ofConcepts in Structure-activity Relationship Studies of Opioid Ligands,”J. Med. Chem., 35:1927-1937 (1992); Portoghese, “Bivalent Ligands andthe Message-address Concept in the Design of Selective Opioid ReceptorAntagonists,” TIPS, 10:230-235 (1989); Shimohigashi et al., “DimericTetrapeptide Enkephalins Display Extraordinary Selectivity for the DeltaOpiate Receptor,” Nature, 297:333-335 (1982), each of these referencesis hereby incorporated by reference in its entirety).

[0086] Evaluation of yohimbine dimers on human α_(2a)-, α_(2b)-, andα_(2c)- AR expressed stably in Chinese hamster ovary (CHO) cells wasconducted by displacement of radiolabeled rauwolsine using the variousdimers. The binding affinities of yohimbine and these dimers are givenin Table 1 below. [³H]Rauwolscine was used as the radioligand inequilibrium competition binding experiments. Yohimbine was included asthe monovalent compound against which bivalent yohimbines were compared.TABLE 1 Binding affinities (K_(i)) of bivalent yohimbines on humanα_(2a)- and α_(2b)-AR expressed in CHO cells^(a) K_(i) = SEM (nM)^(c)α_(2a)/α_(2b) Compound N Formula^(b) α_(2a)-AR α_(2b)-AR Selectivity^(d)Yohimbine 0.42 ± 0.04 2.01 ± 0.29 4.8 3a 2 C₄₂H₅₂N₆O₄—2HCl—2.5H₂O 26.4 ±7.3  1510 ± 262  57.2 3b 3 C₄₃H₅₄N₆O₄—2HCl—2H₂O  52 ± 3.2  990 ± 85.219.0 3c 4 C₄₄H₅₆N₆O₄—2HCl—2H₂O 15.3 ± 6.2  188.6 ± 38.1  12.3 3d 5C₄₅H₅₈N₆O₄—2HCl—1.8H₂O 1.73 ± 0.26 134.3 ± 38.1  77.6 3e 6C₄₆H₆₀N₆O₄—2HCl—3H₂O 1.35 ± 0.27 166.2 ± 56.7  123.1 3f 7C₄₇H₆₂N₆O₄—2HCL—3H₂O 0.87 ± 0.18 27.6 ± 4   31.7 3g 8C₄₈H₆₄N₆O₄—2HCl—2.2H₂O 0.76 ± 0.12  46 ± 7.6 60.5 3h 9C₄₉H₆₆N₆O₄—2HCl—2.5H₂O 1.25 ± 0.29 44.7 ± 1.9  35.8 3i 10C₅₀H₆₅N₆O₄—2HCl—2.5H₂O 0.39 ± 0.08 18.6 ± 1.6  47.7 3j 12C₅₂H₇₂N₆O₄—2HCl—2.5H₂O 1.18 ± 0.38 18.5 ± 0.57 15.7 #itsCharacterization at Cloned Human Alpha-adrenoceptors,” Bioorg. Med.Chem., 3: 1693-1698 (1995), which is herein incorporated by reference inits entirety.

[0087] Several features are evident from the data in Table 1.

[0088] Firstly, all of the yohimbine dimers displayed equal or loweraffinity than the parent antagonist for the α_(2a) subtype; affinity forthe α_(2b) adrenoceptor was reduced to a much greater extent. The lackof enhanced affinity of a dimer for either subtype may imply that thespacers of these bivalent ligands are not long enough to achieve thebridging of vicinal receptors. This consideration has been the basis forpreparing further bivalent yohimbines of the present invention withlonger spacers which are being evaluated at the α₂-AR subtypes. Thisdecision to prepare the dimers of the present invention with longerspacers was also facilitated by recent reports showing that receptorcluster formation was observed immunomicroscopically for the α_(2a)-AR(Uhlen et al., Pharmacol. Commun., 6, 155 (1995), which is hereinincorporated by reference in its entirety) and that α_(2a)- andα_(2c)-AR can exist as dimers (Maggio et al., “Coexpression Studies withMutant Muscarinic/Adrenergic Receptors Provide Evidence forIntermolecular “Cross-talk” Between G-Protein-linked Receptors,” Proc.Natl. Acad. Sci. USA 90:3103-3107 (1993); Maggio et al., “FunctionalRole of the Third Cytoplasmic Loop in Muscarinic Receptor Dimerization,”J. Biol. Chem., 271:31055-31060 (1996), each of these references isherein incorporated by reference in its entirety).

[0089] Secondly, lower binding affinities were observed for all bivalentyohimbines on the α_(2b) than the α_(2a) subtype. The highly divergentextracellular loops between the α_(2a) and α_(2b) subtype, in contrastto their highly homologous seven transmembrane regions, may haveimparted the differential affinities of yohimbine dimers on the α_(2a)and α_(2b) subtypes (Harrison et al., “Molecular. Characterization ofAlpha1- and Alpha2-adrenoreceptors,” TIPS, 12:62-67 (1991); Aantaa etal., “Molecular Pharmacology of Alpha2-adrenoreceptor Subtypes,” Ann.Med. 27:439-449 (1995), each of these references is hereby incorporatedby reference in its entirety).

[0090] One prominent feature of this extracellular loop diversitybetween α_(2a-) and α_(2b)-AR is the differential distribution of acidicand basic amino acid residues in these loops. Basic residues are in apreponderance over acidic residues at α_(2b)-AR, compared to theirdistributions at the α_(2a)-AR. If it is assumed that the extracellularloop amino acid residues have the same pK_(a) values when they are inthe loop microenvironment and when they are free in the solution, theextracellular loops of the α_(2b)-AR are more positively charged thanthe extracellular loops of the α_(2a)-AR under our biological evaluationcondition (pH=7.4).

[0091] Therefore, the binding of one protonated yohimbine moiety of thebivalent yohimbines at the receptor active site, which is within theseven transmembrane regions (Hieble et al., “Alpha- andBeta-adrenoreceptors: From the Gene to the Clinic,” J. Med. Chem.,38:3415-3444 (1995); Harrison et al., “Molecular Characterization ofAlpha1- and Alpha2-adrenoreceptors,” TIPS, 12:62-67 (1991), each ofthese references is hereby incorporated by reference in its entirety),will inevitably place the second protonated yohimbine moiety in anenvironment where strong electronic repulsion between the protonatedyohimbine moiety and the highly positively charged extracellular loopsof the α_(2b)-AR is expected. This destabilizing electronic effect isexpected to disturb the binding process of the yohimbine at theα_(2b)-AR active site, thus, lower binding affinities were observed.However, on the (α_(2a)-AR, the weak electronic repulsion between themuch less positively charged extracellular loops and the protonatedyohimbine moiety is not expected to strongly disturb the binding of theyohimbine at the receptor active site. Furthermore, α_(2A(a))-AR fromboth native tissues and model cell lines are highly glycosylated at theextracellular N-terminal region (Wilson et al., “Monovalent Cation andAmiloride Analog Modulation of Adrenergic Ligand Binding to theUnglycosylated Alpha2B-adrenergic Receptor Subtype,” Mol. Pharmacol.,39:481-486 (1991); Guyer et al., “Cloning, Sequencing, and Expression ofthe Gene Encoding the Porcine Alpha2-adrenergic Receptor,” J. Biol.Chem., 265:17307-17317 (1990), each of these references is herebyincorporated by reference in its entirety), whereas α_(2B(b))-AR is not(Zeng et al., “Molecular Characterization of a Rat Alpha2B-adrenergicReceptor,” Proc. Natl. Acad. Sci. USA, 87:3102-3106 (1990); Lanier etal., “Identification of Structurally Distinct Alpha2-adrenergicReceptors,” J. Biol. Chem., 263:14491-14496 (1988), each of thesereferences is hereby incorporated by reference in its entirety). It islikely that the complex carbohydrate trees of the α_(2a) subtype shieldthe exposed charge, and thus further attenuate the electronic repulsionbetween extracellular loops and protonated yohimbine. A higher bindingaffinity was thus observed for a yohimbine dimer on the α_(2a) than onthe α_(2b) subtype.

[0092] All yohimbine dimers identified above displayed bindingselectivities for human α_(2a)- versus C_(2b)-ARs expressed in CHOcells, with peak selectivity occurring for 3 e (n=6). Four of thesedimeric compounds, i.e. 3d (n=5), 3e (n=6), 3g (n=8), 3i (n=10)represent the most potent and selective (48-fold to 123-fold) ligandsidentified so far for human α_(2a)- versus α_(2b)-ARs expressed in CHOcells.

Example 3 Binding Selectivities for Yohimbine Dimers on Humanα_(2c)-versus α_(2a)- or α_(2b)-Adrenergic Receptors

[0093] By virtue of the results reported in Example 2 above, anexamination of the selectivity of the yohimbine dimers on (α_(2c)-versus (α_(2a)- or (α_(2b)-ARs was conducted.

[0094] Evaluation of yohimbine dimers on human α_(2b)- and α_(2c)-ARexpressed stably in Chinese hamster ovary (CHO) cells was conducted asdescribed above. The results are present in Table 2 below. TABLE 2Binding affinities of yohimbine and its bivalent analogs on humanα_(2a)-, α_(2b)- and α_(2c)-adrenoceptors (AR) expressed in CHO cellspK_(i) Values Compound α_(2a)-AR α_(2b)-AR α_(2c)-AR Yohimbine 8.52 ±0.04 8.00 ± 0.10 9.17 ± 0.02^(b) n = 2 (3a) 7.03 ± 0.12 5.50 ± 0.07 8.45± 0.12^(b) n = 3 (3b) 6.74 ± 0.03 5.69 ± 0.04 8.24 ± 0.07^(b) n = 4 (3c)7.27 ± 0.15 6.40 ± 0.08 8.08 ± 0.17^(b) n = 5 (3d) 8.21 ± 0.07 6.55 ±0.10 7.97 ± 0.13^(a) n = 6 (3e) 8.35 ± 0.10 6.54 ± 0.16 8.16 ± 0.04^(a)n = 7 (3f) 8.51 ± 0.08 7.24 ± 0.07 8.47 ± 0.11^(a) n = 9 (3g) 8.40 ±0.12 7.03 ± 0.02 8.63 ± 0.10^(a) n = 18 (3k) 7.34 ± 0.10 6.60 ± 0.048.43 ± 0.01^(b) n = 24 (3j) 6.39 ± 0.10 5.41 ± 0.02 8.30 ± 0.10^(b)

[0095] As can be seen in Table 2, all of the bivalent yohimbine analogspossess lower binding affinities on each of the three subtypes ascompared to yohimbine. However in a closer evaluation, the dimericanalogs show much higher affinities for the. α_(2a)- and α_(2c)-ARsubtypes versus the α_(2b)-AR at all linker lengths (from n=2 to n=24),and there were relatively small changes in the pKi values for theseanalogs on the (α_(2c)- AR subtype. If a comparision is made between thepKi values of the bivalent yohimbine analogs on the three α₂-ARsubtypes, there is a high degree of selectivity of yohimbine dimers forthe α_(2c)-AR subtype at spacers on n=2, 3, 4, 18 and 24 (see FIG. 1).In particular, it has been shown that the yohimbine dimer (n=24) bindswith a potency for the α_(2c)-AR which is 81- and 776-fold greater thanits binding to the α_(2a)- and α_(2b)-ARs, respectively. It isparticularly noteworthy that the addition of the methylene spacerlinkages has produced analogs that are highly potent and selectiveα_(2c)-AR ligands.

[0096] Taken collectively, these results indicate the following withregard to bivalent yohimbine compounds of the present invention: (1) Allof the yohimbine dimers have higher affinities at the α_(2a)- andα_(2c)-AR as compared to the α_(2b)-AR subtype; (2) Five of the dimericcompounds, i.e. n=2, n=3, n=4, n=18, and n=24, represent potent andselective ligands for the human α_(2c)-AR subtype; (3) The findingsprovide support for demonstrating that the bivalent analog approach isuseful for developing α_(2c)-AR subtype selective ligands.

Example 4 Binding Selectivities for Yohimbine Dimers on Humanα_(2c)-versus α₁-Adrenergic Receptors

[0097] In addition, selected yohimbine dimers (n=3 and n=24) have beenselected for further study of their potency profile of α₁-AR subtypebinding (see Table 3 below), and functional activities as antagonists ofthe human (α_(2a)- and α_(2c)-AR. TABLE 3 Binding affinities (pK_(i)values) of yohimbine and its selective bivalent analogs (n = 3 and n =24) on the human α₁- and α_(2C)-AR subtypes expressed in HEK and CHOcells, respectively. Compound α_(1a)-AR α_(1b)-AR α_(1d)-AR α_(2c)-ARYohimbine 5.45 ± 0.05 5.52 ± 0.04 5.02 ± 0.01 9.17 ± 0.02 n = 3 4.77 ±0.05 5.24 ± 0.03 5.11 ± 0.05 8.24 ± 0.07 n = 24 4.52 ± 0.01 4.91 ± 0.014.51 ± 0.08 8.30 ± 0.10 # expressed in CHO and HEK cells; andnonspecific binding was measured in the presence of 10 μM of yohimbineand 10 μM phentolamine respectively.

[0098] In order to establish the α₂₋ versus α₁₋AR subtype selectivity ofthe bivalent analogs, it was necessary to determine their bindingaffinities on the remaining α₁-AR subtypes. The data given in Table 3demonstrates that yohimbine and its two bivalent analogs bind with amuch higher potency to the α_(2c)-AR subtype as compared to the threeα₁-AR subtypes. The binding affinities of yohimbine, and the n=3 andn=24 dimers for the α_(2c)-AR subtype were at least 4400-, 1000- and2450-fold greater than those on the α₁-AR subtypes, respectively. Thesefindings confirm the binding selectivity of these ligands for theα_(2c)-AR subtype.

Example 5 Activity of Yohimbine Dimer on Reversal of MedetomidineMediated Inhibition of Forskolin-Induced cAMP Levels in CHO CellsExpressing α₂-Adrenergic Receptor Subtypes

[0099] In order to verify that the observed binding affinities of theyohimbine analogs correlate with the functional responses in the α₂-ARsubtypes, the antagonist effects of yohimbine and the two bivalentselective analogs (n=3 and n=24) have been examined for their reversalof medetomidine mediated inhibiton of forskolin-induced cAMP levels inα_(2a)- and α_(2c)-AR.

[0100] These experiments were conducted as follows: a fixedconcentration of medetomidine (10 nM) was added to block thecAMP-induced increases by forskolin (5 μM). The concentration ofmedetomidine was chosen which produced at least a 50% inhibition of theforskolin alone response in the α₂-AR subtypes expressing CHO celllines. Controls of yohimbine, medetomidine and basal responses (solventcontrol) were included. The concentration-dependent effects of yohimbineand two bivalent analogs (n=3 and n=24) to antagonize the action ofmedetomidine are provided in Table 4 below. The changes in cAMP levelswere assessed by luciferase activities using a 6 CRE-LUC reporter genebioassay. Concentrations of yohimbine, bivalent analog (n=3 carbonlinker) and bivalent analog (n=24 carbon linker) varying from 0.01 to 10μM were used. The conditions of incubation were sequentially: theaddition of yohimbine antagonist for 20 min; addition of medetomidinefor 20 min; the addition of forskolin and incubation for 4 hours. TheEC₅₀ values of the yohimbine analog for the reversal of medetomidineeffect's were calculated using the GraphPad Prism program; and the dataare expressed as the mean ±SEM of n=3-4 experiments. The data of EC₅₀values for the yohimbine analogs are presented in Table 4 below. TABLE 4The blockade of medetomidine effects by yohimbine and dimeric analogs onhuman α_(2a)- and α_(2c)-ARs expressed in CHO cells. EffectiveConcentration-50 (nM ± SEM)^(a) α_(2c)-AR Compound α_(2a)-AR α_(2c)-ARSelectivity Ratio^(b) Yohimbine  32 ± 6.4  3.2 ± 2.6 10.0 YohimbineDimer (n = 3)  660 ± 210 15.8 ± 5.2 41.8 Yohimbine Dimer (n = 24) 3170 ±470  110 ± 20 28.8

[0101] The results in Table 4 show that yohimbine and bivalent analogs(n=3 and n=24) were able to reverse the medetomidine effects on theα_(2c)-AR at lower concentrations than those required for reversal onthe α_(2a)-AR. Yohimbine, and its bivalent analogs, n=3 and n=24 were10-fold, 42-fold and 29-fold more potent at inhibiting the action ofmedetomidine on the α_(2c)-AR versus the α_(2a)-AR subtype,respectively. In addition, the data of these functional studies are ingood agreement with the differences in binding affinities observed withthese drugs on these two α2-AR subtypes.

[0102] In summary, these bivalent yohimbine dimers are highly potent,selective α₂-AR antagonists.

Example 6 Intravital Microscopy Studies on the Mouse Cremaster MuscleUsing Yohimbine

[0103] Intravital microscopy is a powerful technique that permits thereal-time study of microvasculature (MV) responses to drugs or changesin physiological conditions.

[0104] Male C57-BL mice age 7-8 weeks and weighing approximately 20 gwere used for the study. Prior to surgery animals were anesthetized withan i.m. injection of 15 mL of Ketaset (87 mg ketamine/mL+13 mgxylazine/mL). Body temperature was maintained at ˜37° C. by convectiveheating. The animal was supinated and a midline incision was made in theventro-cervical region and the underlying tissues bisected laterally toexpose the trachea. A tracheotomy was performed and the animal intubateddirectly with PE50 tubing to facilitate ventilation, and the woundclosed. The left rear limb was incised to expose the femoralneurovascular bundle, and the femoral vein isolated and catheterizedwith PE10 tubing attached to a syringe of normal saline. Next, theanimal was placed on a specially designed surgery board and the righttesticle oriented over the glass translumination window and irrigationinitiated warmed (˜37° C.) physiological saline (4.3 mM NaHCO₃, 26.4 mMNaCl, 0.9 mM KCl, 0.4 mM CaCl₂, 0.2 mM MgSO₄) over the tissue. Thescrotum was incised medically and the underlying fascia blunt dissectedto expose the testicle. The apical portion of the cremaster muscle ispinned distally on the surgery board and the cremaster muscle incisedalong its long axis to expose the testicle. The testicle was then gentlyforced back into the ingunal canal, allowing the cremaster muscle to bepinned evenly around the translumination window.

[0105] All experiments were carried out using an industrial grademicroscope (Nikon MM-11) with two light sources, bright field (Opti-Quip75 W xenon) and fluorescent (Nikon 150 W mercury). The primary cameraassembly consists of a chilled charged coupled device (CCD) andcontroller (Hamamastu C5985). The secondary camera assembly consists ofa CCD camera (MTI CCD72) in conjunction with an intensifier (MTIGENIISYS). Experiments are viewed on a video monitor and recorded ondigital tapes for off-line processing. The video images are analyzedoff-line using the MetaMorph software (Universal Imaging Co.) on a DellOptiplex GX-1. Calibration of the software was done using a micrometerslide and measurements are defined in μm/pixel. An average of 5-7measures per time point per vessel were made and, when possible, thetime interval between measurements is 5 seconds or less for the firstminute of the experiment.

[0106] The prepared animal was placed on the stage of the microscope oneither the surgery board with the translumination window or on astereotaxic frame. Exposed tissues were irrigated with warmedphysiological saline. The tissues were allowed to equilibrate untilnormal blood flow was observed, during this time it is scanned at 10×for A1-A4 arterioles. The arterioles were tested for physiological toneby the administration of 0.1 mM adenosine. Vessels that did notdemonstrate at least a 20% increase in diameter will not be consideredfor the experiment. After the arteriole returns to resting diameter (˜10min), yohimbine was administered systemically using the femoral vein.The blood pressure was monitored with a Kent Scientific XBP-1000 unit.

[0107] Preliminary intravital studies at 37° C. on the cremaster musclearterioles (A3-A4 resistance vessels) of male C57-BL mice have shown indetail the microvascular response to the treatment of animals withyohimbine. Administration of a vehicle solution did not affect thediameter of the observation vessel; however, subsequent treatment withyohimbine resulted in a 22.14±1.08 (n=6) percent increase in vesseldiameter (FIG. 2). It is believed that the yohimbine dimers willsimilarly achieve this result.

[0108] The functional results observed are consistent with either theblockade of presynaptic or an interaction with post-synaptic α₂-ARs.Therefore, preparations are being made to cool the tissue to 28° C. toevaluate if the resting diameter of the resistance vessels decreases,i.e., indicating the possible induction of α_(2c)-ARs. It is believedthe yohimbine dimers will antagonize the effects of temperaturereduction.

[0109] Discussion

[0110] Based upon molecular modeling studies, it is believed thatseveral yohimbine dimers (n=18 or 24) have one pharmacophore binding atthe regular binding site for yohimbine and the second pharmacophorebinding to an adjacent site, possibly one of the extracelluar loops, onthe same α₂-AR. The selectivity may arise from the interaction of one ofthe pharmacophores with the regular binding site and the otherpharmacophore binding with one of the extracellular loops within thesame receptor rather than binding with two different receptors asoriginally believed in the art. Studies conducted with yohimbinecompounds of the present invention examine the important make up ofbivalent molecules to bind with one AR receptor (short linker), asobserved with the yohimbine dimers of n=2, 3 and 4; and to two receptorsby dimers with n=18 and 24 or greater (long linkers). Yohimbine dimercompounds of the present invention have been synthesized with longerlinkers (wherein the integer n=20 and 36) and studied for interactionbetween AR receptors.

[0111] Based on molecular modeling docking studies of yohimbine, it isbelieved that there is an unexplored hydrophobic pocket in the receptor.At present it is unclear as to the exact mechanism underlying theselectivity; however, considering the similarity in the ligand bindingdomains and limited accessible space within the pocket, it is proposedthat specific interactions with the extracellular loops plays a criticalrole in the selectivity. Interestingly, the chain length linking theyohimbine nucleus has only modest effects on α_(2c)-AR selectivity;however, it appears to play a significant role in discriminating betweenon α_(2c)-AR and the α_(2a)- and (α_(2b)-AR subtypes. Based on theseresults, it is believed that extracellular loops to further enhance thereceptor subtype selectivity by utilizing the extracellular loops as theaddress section on the novel dimer molecules.

[0112] Based on an amino acid alignment of the EL residues (Lomasney etal., “Molecular Biology of a-Adrenergic Receptors: Implications forReceptor Classification and for Structure-Function Relationships,”Biochim. Biophys. Acta. 1095:127-139 (1991); Dohlman et al., “A Familyof Receptors Coupled to Guanine Nucleotide Regulatory Proteins,”Biochemistry, 26:2657-2664 (1987), each of these references is herebyincorporated by reference in its entirety) and an analysis of thecharged residues in these regions, it is believed that: (1) the chainlengths n=18 or greater atoms position the second yohimbine moleculeinto the ELs or into an adjacent receptor; (2) the extracellular domainof α_(2b)-AR has a net positive charge (+8) distributed throughout theEL region and, therefore, the second yohimbine molecule (positivelycharged) is repulsed by the EL region to afford reduced bindingaffinity; and (3) the net negative charge of the α_(2c)-AR (−2) combinedwith 10 hydrophobic residues (EL2 and 3) creates an ideal bindingenvironment for the second yohimbine molecule. The profile for theα_(2c)-AR and α_(2a)-AR are less readily analyzed.

[0113] Interestingly, the chain length linking the yohimbine nucleus hasonly modest effects on α_(2c)-AR binding affinity; the pK_(i)'s onlyvaried between 7.97 to 8.63 (Table 2); however, it appears to play asignificant role in discriminating between α_(2c)-AR versusα_(2a- and α) _(2b)-AR subtypes. A qualitative analysis of thehydrophobic residues in EL2 (Gly¹⁹⁶-Leu²⁰⁴) and EL3 (Ala⁴¹⁰-Phe⁴¹⁸) inthe α_(2c)-AR compared to those of the α_(2a)- and α_(2b)-AR (where EL2and EL3 are interspersed with charged residues) indicates an importantrole in the binding of the yohimbine dimers to the α_(2c)-AR (EL1 wasexcluded from the analysis based on the high sequence homology). Basedon the EL analysis presented, a further enhancement of receptor subtypeselectivity will be achieved by utilizing the charge negative, positive,or neutral message-addresses in such novel molecules of the presentinvention.

[0114] Although preferred embodiments have been depicted and describedin detail herein, it will be apparent to those skilled in the relevantart that various modifications, additions, substitutions, and the likecan be made without departing from the spirit of the invention and theseare therefore considered to be within the scope of the invention asdefined in the claims which follow.

What is claimed is:
 1. A method of treating or preventing an α₂adrenergic receptor mediated condition or disorder comprising: providinga compound according to formula (I)

wherein R is a linker molecule that affords activity of the compound asan α₂ adrenergic receptor antagonist; and administering to a patient aneffective amount of the compound to treat or prevent the α₂ adrenergicreceptor mediated condition or disorder.
 2. The method according toclaim 1, wherein the α₂ adrenergic receptor mediated condition ordisorder is an α_(2a) adrenergic receptor mediated condition ordisorder.
 3. The method according to claim 2, wherein the α_(2a)adrenergic receptor mediated condition or disorder is selected from thegroup consisting of hypertension, hypotension, erectile dysfunction,pain, glaucoma, alcohol and drug withdrawal, rheumatoid arthritis,ischemia, migraine, cognitive deficiency, spasticity, diarrhea, andnasal congestion.
 4. The compound according to claim 2, wherein thecompound exhibits selectivity in binding an α_(2a) adrenergic receptorover an α_(2b) adrenergic receptor.
 5. The method according to claim 1,wherein the α₂ adrenergic receptor mediated condition or disorder is anα_(2c) adrenergic receptor mediated condition or disorder.
 6. The methodaccording to claim 5, wherein the α_(2c) adrenergic receptor mediatedcondition or disorder is Raynaud's disease.
 7. The method according toclaim 5, wherein the compound exhibits selectivity in binding an α_(2c)adrenergic receptor over an α_(2b) adrenergic receptor.
 8. The methodaccording to claim 1, wherein R has a length of about 2.5 Å to about 45Å.
 9. The method according to claim 8 wherein R has a length of about2.5 Å to about 5 Å.
 10. The method according to claim 8 wherein R has alength of about 23 Å to about 29 Å.
 11. The method according to claim 1,wherein R is either: (i) a straight or branched chain alkyl, alkenyl,alkynyl comprising at least 2 carbon atoms in a main chain thereof, or(ii) a straight or branched chain alkyl, alkenyl, alkynyl comprising atleast 2 carbon atoms in a main chain thereof and an X group within themain chain and/or a Y group as a substituent linked to a carbon atom inthe main chain, with X being —O—, carbonyl, —NR¹— with R¹ being H or analkyl, —C(O)NHR¹— with R¹ being an alkyl, —S—, sulfoxide, sulfonyl, or acyclic or multicyclic ring with or without hetero atoms as ring membersand including, optionally, one or more substitutions on the ringstructure, and with Y being —OH, —NO₂, —CN, —C(O)H, —SH, a primary,secondary, or tertiary amine, a carboxylic acid, an ester, a keto group,—SO₂NH₂, or —SO₂NHR² with R² being an alkyl, or (iii) a cyclic ormulticyclic ring with or without hetero atoms as ring members andincluding, optionally, one or more substitutions on the ringstructure(s).
 12. The method according to claim 11, wherein R is astraight chain alkyl.
 13. The method according to claim 12, wherein R isa straight chain C2 to C36 alkyl.
 14. The method according to claim 12,wherein R is a straight chain C3 to C24 alkyl.
 15. The method accordingto claim 11, wherein R is a straight chain alkyl comprising at least 4carbon atoms and an X group within the straight chain, wherein X is a—O—.
 16. The method according to claim 15, wherein R is—(CH₂CH₂—O—CH₂CH₂)_(n) wherein n is an integer from 1 to
 6. 17. Themethod according to claim 15, wherein R is—(CH₂CH₂—O—CH₂CH₂—O—CH₂CH₂)_(n)— wherein n is an integer from 1 to 4.18. The method according to claim 11, wherein R is a straight chainalkyl comprising at least 5 carbon atoms and an X group within thestraight chain, wherein X is —C(O)NHR¹— with R¹ being an alkyl.
 19. Themethod according to claim 11, wherein R is—(CH₂—NHC(O))_(n)—CH₂—CH₂—CH₂—CH₂—(C(O)NH—CH₂)_(n)— wherein each n isindependently an integer from 1 to
 3. 20. The method according to claim11, wherein R is a straight chain alkenyl comprising at least 5 carbonatoms in a main chain thereof and an X group within the main chain,wherein X is —C(O)NHR¹— with R¹ being an alkyl.
 21. The method accordingto claim 20, wherein R is a cis isomer or a trans isomer of—(CH₂—NHC(O))_(n)—CH₂—CH═CH—CH₂—(C(O)NH—CH₂)_(n)— wherein each n isindependently an integer from 1 to
 3. 22. The method according to claim21, wherein R is a cis isomer.
 23. The method according to claim 21,wherein R is a trans isomer.
 24. A method of modulating the activity ofan α_(2a) adrenergic receptor comprising: providing a compound accordingto formula (I)

wherein R is a linker molecule that affords activity of the compound asan α_(2a) adrenergic receptor antagonist; and contacting an α_(2a)adrenergic receptor with the compound under conditions effective tomodulate the activity of the α_(2a) adrenergic receptor.
 25. The methodaccording to claim 24, wherein the compound exhibits selectivity inbinding an α_(2a) adrenergic receptor over an α_(2b) adrenergicreceptor.
 26. The method according to claim 24, wherein R has a lengthof about 2.5 Å to about 45 Å.
 27. The method according to claim 24,wherein R is either: (i) a straight or branched chain alkyl, alkenyl,alkynyl comprising at least 2 carbon atoms in a main chain thereof, or(ii) a straight or branched chain alkyl, alkenyl, alkynyl comprising atleast 2 carbon atoms in a main chain thereof and an X group within themain chain and/or a Y group as a substituent linked to a carbon atom inthe main chain, with X being —O—, carbonyl, —NR¹— with R¹ being H or analkyl, —C(O)NHR¹— with R¹ being an alkyl, —S—, sulfoxide, sulfonyl, or acyclic or multicyclic ring with or without hetero atoms as ring membersand including, optionally, one or more substitutions on the ringstructure, and with Y being —OH, —NO₂, —CN, —C(O)H, —SH, a primary,secondary, or tertiary amine, a carboxylic acid, an ester, a keto group,—SO₂NH₂, or —SO₂NHR² with R² being an alkyl, or (iii) a cyclic ormulticyclic ring with or without hetero atoms as ring members andincluding, optionally, one or more substitutions on the ringstructure(s).
 28. The method according to claim 27, wherein R is astraight chain alkyl.
 29. The method according to claim 27, wherein R isa straight chain alkyl comprising at least 4 carbon atoms and an X groupwithin the straight chain, wherein X is a —O—.
 30. The method accordingto claim 27, wherein R is a straight chain alkyl comprising at least 5carbon atoms and an X group within the straight chain, wherein X is—C(O)NHR¹— with R¹ being an alkyl.
 31. The method according to claim 27,wherein R is —(CH₂—NHC(O))_(n)—CH₂—CH₂—CH₂—CH₂—(C(O)NH—CH₂)_(n)— whereineach n is independently an integer from 1 to
 3. 32. The method accordingto claim 27, wherein R is a straight chain alkenyl comprising at least 5carbon atoms in a main chain thereof and an X group within the mainchain, wherein X is —C(O)NHR¹— with R¹ being an alkyl.
 33. The methodaccording to claim 32, wherein R is a cis isomer or a trans isomer of—CH₂—NHC(O))_(n)—CH₂—CH═CH—CH₂—(C(O)NH—CH₂)_(n)—wherein each n isindependently an integer from 1 to
 3. 34. A method of modulating theactivity of an α_(2c) adrenergic receptor comprising: providing acompound according to formula (I)

wherein R is a linker molecule that affords activity of the compound asan α_(2c) adrenergic receptor antagonist; and contacting an α_(2c)adrenergic receptor with the compound under conditions effective tomodulate the activity of the α_(2c) adrenergic receptor.
 35. Thecompound according to claim 34, wherein the yohimbine dimer exhibitsselectivity in binding an α_(2c) adrenergic receptor over an α_(2b)adrenergic receptor.
 36. The method according to claim 34, wherein thecompound exhibits selectivity in binding an α_(2c) adrenergic receptorover an (α_(2a) adrenergic receptor.
 37. The method according to claim34, wherein R has a length of about 2.5 Å to about 45 Å.
 38. The methodaccording to claim 34, wherein R is either: (i) a straight or branchedchain alkyl, alkenyl, alkynyl comprising at least 2 carbon atoms in amain chain thereof, or (ii) a straight or branched chain alkyl, alkenyl,alkynyl comprising at least 2 carbon atoms in a main chain thereof andan X group within the main chain and/or a Y group as a substituentlinked to a carbon atom in the main chain, with X being —O—, carbonyl,—NR¹— with R¹ being H or an alkyl, —C(O)NHR¹— with R¹ being an alkyl,—S—, sulfoxide, sulfonyl, or a cyclic or multicyclic ring with orwithout hetero atoms as ring members and including, optionally, one ormore substitutions on the ring structure, and with Y being —OH, —NO₂,—CN, —C(O)H, —SH, a primary, secondary, or tertiary amine, a carboxylicacid, an ester, a keto group, —SO₂NH₂, or —SO₂NHR² with R² being analkyl, or (iii) a cyclic or multicyclic ring with or without heteroatoms as ring members and including, optionally, one or moresubstitutions on the ring structure(s).
 39. The method according toclaim 38, wherein R is a straight chain alkyl.
 40. The method accordingto claim 38, wherein R is a straight chain alkyl comprising at least 4carbon atoms and an X group within the straight chain, wherein X is a—O—.
 41. The method according to claim 38, wherein R is a straight chainalkyl comprising at least 5 carbon atoms and an X group within thestraight chain, wherein X is —C(O)NHR¹— with R¹ being an alkyl.
 42. Themethod according to claim 38, wherein R is—(CH₂—NHC(O))_(n)—CH₂—CH₂—CH₂—CH₂—(C(O)NH—CH₂)_(n)— wherein each n isindependently an integer from 1 to
 3. 43. The method according to claim38, wherein R is a straight chain alkenyl comprising at least 5 carbonatoms in a main chain thereof and an X group within the main chain,wherein X is —C(O)NHR¹— with R¹ being an alkyl.
 44. The method accordingto claim 43, wherein R is a cis isomer or a trans isomer of—(CH₂—NHC(O))_(n)—CH₂—CH═CH—CH₂—(C(O)NH—CH₂)_(n)— wherein each n isindependently an integer from 1 to 3.